Toric or toroidal-type traction drive transmissions transmit torque from one rotating semi-toroidal member or disc to another semi-toroidal member or disc by roller members through a traction force. The roller members are supported on a traction fluid oil film through which the traction force is transmitted. A force which creates a normal force between the rollers and the discs urges the discs toward each other. The traction force divided by the normal force defines a coefficient of traction. The rollers and discs have a maximum allowable normal force, determined by Hertzian contact stresses, which limits the input torque to the toric drive. The input torque to the transmission will be maintained below a value determined by the Hertzian contact stresses such that the normal force is within a safe range.
The maximum input torque equals the product of the radius of the contact point on the input disc, the normal force and the maximum coefficient of traction of the traction fluid. If the input torque exceeds this value, detrimental slippage will occur between the discs and the rollers which can significantly damage the transmission. To prevent this from occurring, the normal force and traction force must be controlled to maintain the coefficient of traction below the maximum allowable value.
Traction drives have employed various strategies to control the normal force between the rollers and the discs. One such strategy is to apply hydraulic pressure to force the discs axially toward the roller. This permits much flexibility in controlling the ratio of normal force to traction force. However, this requires a control system that can determine the input torque, the speed ratio of the traction drive and the maximum possible coefficient of traction for all operating conditions.
Another strategy employs a cam loading system which operates on the input disc to apply an axial force which is a linear function of the input torque. This strategy does not need to have a separate input torque measurement; however, this does not compensate for changes in speed ratio. This results in the system being operated at a sub-optimal coefficient of traction at all but one operating condition.
U.S. Pat. No. 5,607,372 describes a toric drive system which improves the hydraulic control of the normal force. This system directs hydraulic pressure to react against a traction force piston and employs the same pressure level to apply an axial force piston to the discs. While this ensures that the ratio of traction force to normal force is constant, it does not compensate for the steer angle of the rollers which varies as a function of the cosine of the roller steer angle.